Skip to main content

Historic Preservation - Technical Procedures

Spectitle:

Repairing Cracks In Architectural Scagliola

Procedure code:

0920013R

Source:

Interior's Handbook For Historic Buildings - Jeff Greene

Division:

Finishes

Section:

Lath & Plaster

Last Modified:

02/24/2012

Details:

Repairing Cracks In Architectural Scagliola



REPAIRING CRACKS IN ARCHITECTURAL SCAGLIOLA


PART 1---GENERAL

1.01 SUMMARY

    A.   This procedure includes guidance on stitching cracks in
         architectural scagliola caused by movement between two
         panels.  The stitching of cracks is typically performed
         when it is not possible or practical to remove and reset
         the panels, or face-pin the pieces.

         NOTE: THIS IS A VERY SPECIALIZED AND LABOR INTENSIVE
         PROCEDURE.  IT SHOULD ONLY BE PERFORMED BY AN EXPERIENCED
         CONSERVATOR.

    B.   For general information on scagliola, including its
         characteristics, uses and problems, see 09200-05-S.

    C.   See 01100-07-S for general project guidelines to be
         reviewed along with this procedure.  These guidelines
         cover the following sections:

         1.   Safety Precautions

         2.   Historic Structures Precautions

         3.   Submittals

         4.   Quality Assurance

         5.   Delivery, Storage and Handling

         6.   Project/Site Conditions

         7.   Sequencing and Scheduling

         8.   General Protection (Surface and Surrounding)

         These guidelines should be reviewed prior to performing
         this procedure and should be followed, when applicable,
         along with recommendations from the Regional Historic
         Preservation Officer (RHPO).


PART 2---PRODUCTS

2.01 MATERIALS

    A.   Keene's Industrial Quick Set Cement

    B.   Limeproof fresco pigments

    C.   Sandpaper (must be glued to sanding or blocks)

    D.   Clean, potable water

    E.   Clean, soft cotton cloths

2.03 EQUIPMENT

    A.   Electric dremel tool or pneumatic die grinder


PART 3---EXECUTION

3.01 EXAMINATION

    A.   Verification of Conditions:

         1.   Determine the source of the crack.  Is there
              evidence of movement between sections?

         2.   Determine which type of scagliola it is - true scag
              or marezzo.  THIS WILL REQUIRE A CONSERVATOR'S
              EXPERTISE.  The biggest difference lies in how each
              is manufactured, applied and finished.  Marezzo is
              made in reverse order from the way true scag is
              produced and is generally a less labor-intensive
              process.  Recognizing the difference between the
              two can aid in better understanding the problem or
              failure.  The typical polish used for each type is
              also significant, as some polishes have proven to
              be detrimental to the material.

3.02 PREPARATION

    A.   Surface Preparation: ALWAYS test compatibility of repair
         mixes, including color and composition.

3.03 ERECTION, INSTALLATION, APPLICATION

    A.   Rout out crack using an electric dremel too or a
         pneumatic die grinder.

    B.   Channel crack in an inverted "V".

    C.   Cut slots into crack - about 1" long and 1/2 the depth of
         material to be fixed.

    D.   Inject epoxy adhesive into slot and insert headless
         stainless steel screw into slot.  Leave approximately
         3/8" space on top for backing and touching in.

    E.   Touching In:

         1.   Prepare plaster mix using Keene's Industrial Quick
              Set Cement and limeproof pigments.  Mix enough for
              the entire touch in.

              a.   DO NOT MIX THE PLASTER INTO THE WATER.  Put
                   the water in the container and sprinkle the
                   plaster around the edges.  

              b.   Carefully mix in selected pigments.  DO NOT
                   BEAT THE MIXTURE, as this accelerates the
                   setting time.

         2.   Set a small amount of mix aside for use later.

         3.   Thoroughly wet the surrounding area of scagliola to
              be treated.

         4.   Backfill hole or crack with scrim and white plaster
              (Keene's cement) to about 3/16" to 1/8" from the
              surface.

         5.   Mix dry pigment into the plaster mix on the palette
              to achieve the desired color.

         6.   Trowel pigmented plaster into the crack or hole to
              match the existing surfaces.  

         7.   To give the appearance of veins:

              a.   Pull a needle threaded with pigmented silk
                   through the mix.

              b.   Build up plaster 1/8" above the surface.
                   Allow to cure.

              c.   Sand the surface with wet or dry sandpaper so
                   that it is uniform with the surrounding
                   surfaces.

              d.   Wipe the surface clean using clean, soft
                   cotton cloths.

              e.   Fill small voids in the surface by "stopping
                   back":

                   1)   Apply a very thin mixture of plaster over
                        the surface; mixture should be pigmented
                        a neutral color.

                   2)   Before the mixture cures, strike it off
                        in a vertical direction using a wood or
                        plastic spatula. Small fissures should
                        remain filled from this process.

                   3)   Rewet the surface with clean, clear
                        water.

                   4)   If fissures were not completely filled,
                        repeat the process of "stopping in".
                        This time, strike off in a horizontal
                        direction.

                   5)   Rinse the surface again with clean, clear
                        water and dry with clean, cotton cloths.

                   6)   Carefully sand the surface again.

         8.   For guidance on polishing the scagliola surface,
              see 09200-07-R.

                         END OF SECTION